555 timer circuit error

Question

I have recently built a circuit with a 555 set in monostable mode which was supposed to turn on three bright LEDs for 51 seconds when a button was pressed.The button was connected with some long wires.After I built it,I decided to connect the battery and was surprised to see that the LEDs turned on even if nobody was pushing the button.I connected and disconnected the battery several times and it kept happening.

^{simulate this circuit – Schematic created using CircuitLab}

It worked poorly in this configuration and after I added a resistor between the positive rail and the trigger pin it seemed to work fine,but I didn't check it too much.What made the IC go mad?Did the long wires make it sensible to static electricity?Was the missing resistor causing the problem?

Answer 1

The missing resistor (known as a pull-up resistor) was the issue. Without it, there is nothing to define the voltage at the trigger pin, except when the button is pressed. This means that airborne electrical noise can couple in to the pin, which might go low enough to trigger the circuit. Since the circuit then latches on for a long time, it doesn't matter that the random noise might then go to a higher voltage again - one very short low trigger is enough to produce the behaviour you saw.

Long wires will definitely have increased the circuit's ability to pick up this random noise by acting as a better aerial (although not "better" for your circuit).

Answer 2

You need a pull-up resistor on the trigger input of the 555 timer. In addition, because you are using long wires, I'd also add a RC filter to reduce the effect of noise pickup.

^{simulate this circuit – Schematic created using CircuitLab}

Note that the RC filter doesn't have any negative effect on the circuit - both the Threshold and Trigger inputs are high-impedance. But without the RC filter, impulse noise such as static discharge can trigger the timer.